With respect to Barca's MF, I read about some rumors regarding Rabiot. But, like
@speedracer said, it would make a lot of sense to see if Roberto can finally be a rotation piece (really a backup for Busi) in the MF. There are also a few academy players that may finally make the jump to fulltime squad (Alena, Arnaiz, and to a lesser extent Samper, not to mention guys that are getting good press but are blocked like Cucurella and Palencia - not MF though). Arthur seems like he will get some real PT, but it's hard to say with South American imports (see: Douglas). Rafinha is still on the squad, but should be loaned or sold. Gomes is likely to be sold and Arsenal makes too much sense.
On defense, there is still some player movement that will occur. Digne has indicated that he wants to leave, although Palencia and Cucurella may be ready for backup duty. I actually really like Digne and if he can wait Alba out one more season, he could be the heir apparent (and Alba could use more rest). Mina is almost certain to go out on loan with the excellent Llenglet coming in. Umtiti will be the no doubt starter and Pique will still get tons of minutes, but I hope he gets more rotation. I think Vermaelen is still on the squad, but may not be come fixture 1 in which case, Mina would stay.
The biggest issue I see at the moment though is a backup CF. If it's Alcacer, there needs to be enough confidence in him, Dembele, and Coutinho to keep the offense in top shape when Suarez is off form and Messi needs rest (something I think will become a big issue this year finally, similar to CR7 last season). There are a few CF targets that make sense to me: Angel Correa (not much of a CF though), Lewa (would make a lot of sense), Morata (unlikely), Rodrigo, Immobile, Belotti, Kramaric (I think he plays CF for Hoffenheim), Mertens (if he plays CF for Napoli), Ben Yedder, Maxi Gomez, Dzeko (as a stop gap), Alario, Milik, etc. It's been the Barca MO in recent years to just go for the best available regardless of long term plans, which makes me think Lewa could be possible (total speculation on my part based on his barking at Bayern), but Barca are always apt to raid other La Liga teams too so Rodrigo and Gomez make sense too. On the wings, Messi, Dembele, Coutinho is not quite enough for rotation, especially if Couts is going to be used in MF. Vidal and Munir are still on the squad but not up to quality IMO. I hope Denis gets some action, or Arnaiz gets a shot. I could still see a quality but not headlining deal here (similar to Semedo last year).